Why circular polarizers?

Okay. I think I got it now. It all has to do with the auto-focus mechanism and not the sensor. That had never occurred to me. Many thanks for all your nice replies.

I don't see how polarization effects can do much to a phase-detect autofocus; there might be some slight birefringence effects but those should be minimal. Phase-detect depends on comparing the position of two features, which is not affected by polarization.

What does get bothered a lot is the metering system, and autofocus is the reason for that.

A phase-detect autofocus needs to see some light when the mirror's down. So the reflex mirror in an autofocus SLR is partially transparent; the light that gets through the mirror is sent to the autofocus. The metering is up in the pentaprism.

Now, the transmission of a partially-reflective mirror depends on the polarization of the light that's hitting it. It transmits a lot more light with P-polarization than with S-polarization (google those terms for some pictures). But if more light goes through the mirror than it should, the meter calibration becomes incorrect. So a linear polarizer can give you exposure errors.